Woolmer and McCulloch [1] describe the topology of a Y machine, discussing its advantages of reduced iron in the stator enabling an improvement in torque density. It comprises a series of coils wound around bars spaced circumferentially around the stator, ideally axially disposed, (ie parallel the rotation axis of the rotor). The rotor has two stages comprising discs provided with permanent magnets that face either end of each coil of the stator. The magnetic path at any stage of operation is: through a first coil into a first magnet on a first stage of the rotor; across a back iron of the rotor to an adjacent second magnet on the first stage; through a second coil of the stator adjacent the first coil; into a first magnet on the second stage of the rotor aligned with the second magnet on the first stage; across the back iron of the second stage to a second magnet on the second stage and aligned with the first magnet on the first stage; and completing the circuit through the first coil.
One difficulty with electric machines generally is to provide adequate cooling. This is a particular problem with a Y machine having a high torque density that significant heat is generated in the coils at high torques and is often a limiting factor in the torques that can be employed, at least for extended periods of time.
Magnetic connection between the coils and the permanent magnets depends on a strong magnetic field being developed through the coils, either by the magnets in the case of a generator or by the coils themselves in the case of a motor and the permeability of the magnetic circuit should be as low as possible to permit the maximum flux density through the coils. For this purpose a high permeability core or bar is provided around which the coils are wound. However, the bar is preferably laminated or otherwise arranged to reduce the incidence of eddy currents in the bar. Also, the bars are preferably provided with shoes to spread the flux in the air gap and reduce the flux density therein—the air gap is of high reluctance and increasing its area reduces that reluctance, and that means that less permanent magnet material can be used. It is desirable to reduce the amount of such material to a minimum.
WO-A-2006/066740 discloses a Y machine comprising a housing having a cylindrical sleeve mounting stator coils internally, the sleeve being hollow whereby cooling medium is circulated. However, the coils are embedded in a thermally conducting material to carry heat to the stator housing. A rotor is rotatably journalled in the housing. The stator bars appear to be laminated, as they are in GB-A-2379093 that also discloses a Y machine, as does WO-A-03/094327.
U.S. Pat. No. 6,720,688 discloses a Y machine in which the rotor acts as a vane pump to circulate fluid within a chamber defined by a stator housing through which a rotor shaft, supported on bearings in the housing and carrying the rotor, extends. The fluid cools stator coils. US-A-2005/0035676 discloses another Y machine, particularly adapted for gearless drive of a vehicle wheel.
US-A-2007/0046124 discloses a Y machine in which the rotor has two circumferentially arrayed rows of alternating segments of permanent magnets and ferromagnetic pole pieces.
Copending international patent applications, publication numbers WO-A-2010/092400 (“A1”), WO-A-2010/092403 (“A2”) and WO-A-2010/092402 (“A3”), disclose respectively various aspects of cooling, flux management and modular arrangement of Y machines. The entire contents of each application is incorporated herein by reference.
A1 discloses an electric machine comprising a rotor having permanent magnets and a stator having coils wound on stator bars for interaction with the magnets across an air gap defined between them, wherein the bars and coils thereon are enclosed by a stator housing that extends between the air gap and defines a chamber incorporating cooling medium to cool the coils.
The stator housing may comprise two annular plates and two cylindrical walls, the annular plates including recesses to locate the bars within the chamber. Preferably, the material of the stator housing is non-magnetic and non-conducting. However, in the case of the separate annular plates and cylindrical walls, said cylindrical walls are preferably aluminium and said annular plates are plastics material. Alternatively, said annular plates may be integral with said cylindrical walls, which cylindrical walls are in this case split around their circumference and connected together along inner and outer circumferential seams. The split may be central defining two clamshells. The clamshells may be substantially identical, possibly being “mirror” images so that they fit one another, facilitating seam welding around the joins at the splits. The clamshells may be plastics.
The annular plates may be thinned at the ends of the bar to minimize the gap between the bars and the magnets on the rotor. The cylindrical walls may be an inner and outer wall, said outer wall having means to mount the machine and said inner walls comprising means to mount bearings for the rotor.
In A2, an axial flux electric machine comprises a rotor having permanent magnets spaced circumferentially on first and second stages of the rotor and a stator disposed between said stages and having coils wound on stator bars of the stator for magnetic interaction with the magnets across an air gap defined between the rotor and stator, wherein the bars have a shoe at each end of each bar that links magnetic flux through the bars with said magnets on each stage, and wherein the stator is a casting of at least two annular components, each comprising a ring of connected shoes and one including some or all the bars or parts of them and the other comprising any remaining bars or parts of them, said coils being disposed on the bars before the annular components are connected together to complete construction of said stator. The annular components may be identical, and may each comprise half of each bar, and be provided with interfaces adapted to facilitate connection.
Also disclosed and described in A2 is an electric machine comprising a rotor having permanent magnets and a stator having coils wound on stator bars for interaction with the magnets across an air gap defined between them, wherein the bars have shoes that link magnetic flux through the bars with said magnets, and wherein the bars and shoes are formed separately from one another and at least a part of each is formed by moulding soft-iron particles so that the particles have a short dimension that is arranged transverse a reluctance-plane, and the bars and shoes are assembled so that said reluctance-plane of the bar is parallel a longitudinal axis of the bar and said reluctance-plane of the shoe is transverse said longitudinal axis.
The alignment of the short dimension of the particles transverse said reluctance-planes results in each reluctance-plane having a minimum reluctance. The particles of at least the bars may have a single longitudinal dimension and said particles may be also aligned so that their longitudinal dimension is parallel a reluctance-direction in said reluctance-plane, said reluctance-direction of the bars being parallel said longitudinal axis of the bar. If the particles of the shoes have a single longitudinal dimension, preferably said reluctance-direction is radial with respect to said longitudinal axis when the bars and shoes are assembled.
In A3, the rotor stages may each comprise an annular dish, whose outer rims mount said permanent magnets and whose inner rims are connected together enclosing said bearings. The rotor stages are dish-shaped to increase their rigidity in a radial plane (ie a plane perpendicular to the rotation axis of the rotor and also, preferably, perpendicular to the stator bars). Each said inner rim may comprise a cylindrical flange with an interface for mutual inter-engagement. A spacer may be provided between the cylindrical flanges to adjust preload on the bearing. The cylindrical flanges can include bosses arranged parallel said rotor axis to receive fasteners for clamping said rotor stages together.
The present invention is concerned with aspects of the clamshell housing of the stator coils, whereby manufacture of the machine is facilitated.
US-A-2006/0043821 discloses over-moulding of components of the machine. This is effected in two stages. First of all, stator bars are over moulded with a bobbin shaped sleeve having flanges on which are then wound coils that are temporarily assembled using a location ring or series of connectors. The assembled bars and coils are inserted into a mould whereupon the entire stator is formed as an annular plate by injection moulding. A similar arrangement is disclosed in WO-A-01/06623.